Modern desktop publishing has made possible the production of one of a kind posters and other displays. A user of a conventional personal computer equipped with any one of a number of inexpensive printers can create professional quality artwork and displays. If the user also has access to a scanner, the user can input almost any type of artwork and modify the artwork for inclusion in the user's own creation.
While the ability to generate custom artwork has advanced rapidly, the ability to "hang" the art work has not progressed so rapidly. The options for displaying the artwork can be more or less summarized as "glue", "frame", or "project". Framing is not cost effective for many temporary displays. In addition, the frame must be mounted on the surface by a fastener or the like. Such fasteners can damage the surface. Such damage is particularly objectionable if the artist is renting the premises at which the damage occurs.
Transparency materials for use with overhead projectors have been available for some time. Unfortunately, one needs a projector for each display. While such systems are practical for presentations to groups in business or educational settings, these systems are far from adequate in terms of serving the needs of an artist or teacher who wishes to hang a picture on a wall. Furthermore, the number of pictures that can be displayed at a time is limited to one per projector.
Printer compatible papers with glue backing are also known to those skilled in the publishing arts. These vary in sizes from small printer labels to full sized sheets. An art work printed on an adhesive backed sheet may be stuck to a surface without the need to frame the art work or damage the surface by the application of fasteners. Unfortunately, such adhesive backed sheets have a number of problems. First, the artwork cannot, in general, be removed from one location and re-applied at another location more than a few times without the adhesive failing. In addition, some of the adhesives used in these sheets leave a residue on the surface. Furthermore, the adhesives used with some of these papers, such as those used for labels, are sufficiently strong that the paper may not be removed from surfaces such as glass without destroying the artwork. In some cases, the art work must be scraped from the surface using a razor blade or the like.
One type of material that holds the promise of providing a display which sticks to a surface without the use of fasteners or adhesives is the electrets. For the purposes of the present discussion, an electret will be defined to be a plastic sheet which carries a permanent electrostatic charge. Such sheets can be generated by passing the plastic sheet through an electric field while the sheet is held at an elevated temperature. The sheet is then cooled before the induced electric diople moment generated by the field has time to dissipate. The resulting plastic sheet will stick to most clean surfaces. Furthermore, the sheet may be easily removed and re-applied to another surface.
Electret compositions for making posters have been described in the prior art. Pads of sheets are commercially sold for use during presentations as a replacement for conventional "white boards". These sheets are typically made from polypropylene which is a few thousandths of an inch thick.
The sheets may be printed via conventional offset printing techniques provided they are re-charged after the printing process. Offset printing systems heat the sheets after the application of the ink to dry the ink. The increase in temperature results in a loss of some or all of the induced dipole moment in the sheet. U.S. Pat. No. 5,477,784 entitled "Apparatus and Method for Printing on and Polarizing Polymer Electret Films" describes a printing arrangement in which the sheets are recharged during the drying process. While such recharging systems may be incorporated into large offset presses, they do not represent a viable solution at the desktop publishing level, since they would require modifications of the existing printers to include high voltage charging electrodes.
In principle, electrets should be capable of use with inkjet printers and some color printers which do not subject the electrets to excessive heat. Unfortunately, the electret sheets present a number of problems with respect to the sheet feeders used in these printers. To be cost effective, and to provide reliable adhesion, relatively thin sheets must be used. These sheets are too flexible to feed properly in many paper handling systems. Furthermore, individual sheets in a stack of electrostatically charged sheets in a sheet feeder tend to stick together because of the electrostatic charge. This further complicates the sheet feeding problem.
Broadly, it is the object of the present invention to provide an improved printing material that incorporates the advantages of electrets.
It is another object of the present invention to provide a printing material that may be printed in conventional computer printers without suffering from the paper feed problems described above.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.